Cavender-Bares, K. K. Massachusetts Institute of Technology, bares@mit.edu
Chisholm, S. W. Massachusetts Institute of Technology, chisholm@mit.edu

 
RESPONSE OF PHYTOPLANKTON GROUPS TO ENRICHMENT WITH DIFFERENT FORMS OF NITROGEN IN THE WESTERN SARGASSO SEA
 
Differential reliance on "recycled" and "new" nutrients may be a factor determining phytoplankton community structure--especially in nutrient deplete surface waters. Using flow cytometry, we monitored the changes in single cell fluorescence of Prochlorococcus, Synechococcus, and the combined ultra- and nanophytoplankton (ultra/nano) group in response to enrichment with nitrate and ammonium; increases in cellular fluorescence were attributed to a release from physiological nutrient limitation. Both Synechococcus and the ultra/nano group showed increases in fluorescence per cell after the addition of ammonium or nitrate, usually within 24 hours. The typical response of Prochlorococcus to enrichment, however, was more complex in that their flow cytometric population differentiated into two subgroups soon after enrichment; fluorescence per cell decreased in one group, whereas in the other it matched or surpassed the control. This behavior was generally independent of the form N added and occurred in spite of trace metal clean methods, which included chelexed nutrient stocks. We hypothesize that these sub-populations reflect physiologically and genetically distinct ecotypes, which are in balance before the nutrient additions. The complex changes invoked by nutrient enrichment and bottle incubations--involving food web structure and nutrient interactions-appear to disrupt this balance.
 
Day: Friday, Feb. 5
Time: 11:15 - 11:30am
Location: Sweeney Center
 
Code: CS55FR1115S